1. Field of the Invention
The present invention relates to antifuse technology. More specifically, the present invention relates to methods for programming antifuse devices.
2. The Prior Art
Antifuse devices are known in the art. Antifuse devices comprise a pair of conductive electrodes separated by at least one layer of antifuse material and may include one or more diffusion barrier layers. Prior to programming, antifuses exhibit very high resistance between the two electrodes and may be considered to be open circuits. A programming process disrupts the antifuse material and creates a low-impedance connection between the two conductive electrodes.
Antifuses are generally classifiable in two categories. A first type of antifuse has a doped region in a semiconductor substrate as its lower electrode and a layer of metal or doped polysilicon as its upper electrode. The antifuse material comprises one or more layers of silicon nitride or silicon dioxide. This type of antifuse is referred to as a substrate antifuse.
A second type of antifuse has a first metal layer disposed above and insulated from a semiconductor substrate as its lower electrode and a second metal layer as its upper electrode. The antifuse material comprises a layer of a material such as amorphous silicon and may be accompanied by one or more barrier layers separating it from the first and/or second metal layers. This type of antifuse is referred to as a metal-to-metal antifuse.
Numerous methods for programming antifuses are known in the art. According to a first prior-art antifuse programming method, direct-current programming potential is applied across the conductive electrodes for a period of time sufficient to disrupt the antifuse material and create at least one conductive filament therethrough. The conductive filament is formed from material from either one or both of the conductive electrodes or the barrier material. An illustrative example of such an antifuse programming method is disclosed in U.S. Pat. No. 4,899,205 to Hamdy et al.
According to another prior-art antifuse programming method, a series of programming potential pulses are applied across the electrodes of the antifuse as the resistance of the programmed antifuse is measured. The number of pulses is determined by the measured resistance of the antifuse. An illustrative example of such an antifuse programming method is disclosed in U.S. Pat. No. 5,008,855 to Eltoukhy et al.
Other methods for programming antifuses are known. Illustrative non-exhaustive examples of such programming methods are disclosed in U.S. Pat. No. 5,126,282 to Chiang et al. and U.S. Pat. No. 5,272,388 to Bakker et al.
According to one example of a prior-art antifuse programming method, a programming pulse is applied across the antifuse electrodes. The pulse has a magnitude equal to a programming potential and has its most positive potential applied to the upper electrode of the antifuse. The programming pulse is followed by a soak pulse having a magnitude equal to from between about 50% to about 80% of the magnitude of the programming potential. The most negative potential of the soak pulse is applied to the upper electrode of the antifuse. The sequence of programming and soak pulses are repeated until the antifuse breaks down after which they are repeated a fixed number of times (e.g., five times).
While these prior-art methods have proved to be successful for programming antifuses, there remains room for improvement with respect to both the reliability of the programmed antifuse and the time it takes to perform the programming of the antifuse.
The present invention is a method for programming antifuses. A programming pulse having a magnitude equal to the programming potential is applied across the conductive electrodes of the antifuse such that the more positive potential is applied to the upper electrode of the antifuse. The disruption of the antifuse material is sensed by an increase in the flow of current. The programming pulse is extended for a fixed period of time following the current increase indicating the disruption of the antifuse material. The programming pulse is followed by a soak pulse. The magnitude of the soak pulse is equal to or less than the magnitude of the programming potential. The polarity of the soak pulse is such that the more negative potential is applied to the upper electrode of the antifuse. After the soak pulse, a program verification pulse may be applied to the antifuse to determine if its resistance is low enough. The magnitude of the programming verification pulse is less than that of the programming and soak pulses and may preferrably be from about 20% to about 30% of the magnitude of the programming and soak pulses. The polarity of the programming verification pulse may be such that either the more positive or more negative potential may be applied to the upper conductive electrode of the antifuse.